I had just come here to ask exactly the same question. Obviously there is a need to build a CB pluggin for a Blog that works with Joomla 1.5
I think the first question to ask is "what is the Beat blog that is compatible with joomla 1.5". Then work from there.
I'd also like to know what sort of private mesaging works with CB, Joomla 1.5 and Fireboard??